Flowmeter



G. L. BORELL Jan. 4, 1949.

FLOWMETER Filed April 25, 1945 IN VE N T 650065 l.. BOHELL /T/WYPatented Jan. 4, 1949 FLOWMETER George L, Borell, Minneapolis, Minn.,assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn.,a corporation of Delaware Application April 25, 1945, Serial No. 590,227

A4 claas; in: Tg2-wi* l My invention relates generally to flowmeters,and more particularly to such meters wherein a stream of fluid impingesupon a variable' impedance member to change the impedance there; A

of, and to means for compensating such meters'.

In the measurement of the ow of fluid, one method which has often vbeenused with-con@I siderable success'is to direct the fluid, whose flow isto be measured, across a heated resistor and to measure the change inresistance of this resistor produced by the cooling effect of thefluid.'

This method has been quite widely used, and if the conditions of thefluid remain constant. such a meter will measure the velocity of thefluid. It has pre'rlously been suggested to use such a meter inconjunction with a reservoir to measure the rate of climb or descent ofan airplane, but such meters have not found much use since they have notbeen considered sufficiently accurate. I

have found that the principal causes of error inV meter and compensationdevice built in accord-l 2 These and other objects of my invention willbecome apparent from the following description of preferred and modifiedforms thereof and from the drawings illustrating those forms in whichFigure l is a schematic representation of a ance withmy preferreddesign,

to provide a flowmeter which may be compensated for changes in thedensity of the fluid whose flow is being measured.

It is another object of my invention to provide a compensating means forowmeters by which the response of the meter may be varied in accordancewith a condition.

It is an additional object of my invention to provide a compensatingmeans which may be applied to existing flowmeters with little or nochange save that of recalibration.

. It is a further object of my invention to provide a fiowmeterpeculiarly adapted -for use with my compensating means and especiallysuited for aircraft installation.

It is also an object of my invention to provide a compensating meanswhich may produce a linear or nonlinear compensating effect dependingupon the particular type of compensation needed.

It is a still further object of my invention to provide a compensatingmeans which may be adapted to be controlled by any desired condition forwhich compensation must be made.

Figure 2 is a cross-sectional view of my improved sensing device which Iprefer to use with my compensating means, and

Figure 3 is a graph illustrating how the varying density of the airrequires a greater change in altitude to produce a given change ofpressure at a greater altitude.

Referring now to the drawings and particularly to Figure 1 thereof, thenumeral I0 indicates a reservoir of any suitable shape, sealed toprevent the admission or release of air therefrom except through asingle opening II therein. A tube I2 leads from the opening II to a flowmeter I3, and from there connection is made to a source of staticpressure (not shown) so that the interior of the reservoir is maintainedat a pressure very nearly equal to the 4static pressure. As applied toaircraft, the term static pressure" means the pressure of the atmosphereat the particular altitude at which the plane is flying, as opposed tothe dynamic pressure which is a combination of the atmospheric pressureand the pressure due to the forward movement of' the plane through theatmosphere. The customary way of securing this static pressure is by theuse of the Pitot-static tube or head, and since such devices are wellknown in the art, they will not be further described here.

Between the reservoir I0 and the source of static pressure, the tube I2is connected to the owmeter I3 which is provided with a by-pass I4 sothat air in flowing from the reservoir I0 to the source of staticpressure may pass either through the owmeter I3 or through the by-passI4. A tube 34 completes the connection from the reservoir I0 to thesource of static pressure. The flow meter I3 is of the balanced bridge,hot wire type and is electrically connected in a circuit I5, hereinafterdescribed, so that any flow of air through the flowmeter will unbalancethe bridge and give an indication of the direction and amount of flow ofair.

As shown in Figure 1, the circuit I 5 may consist of a very simplebridge circuit with a resistor I6, having a center tap I'I, connected toany suitable source of power, such as the planes batteries, or atransformer I8 which is energized by the planes inverter (not shown).Within the owmeter I3 are a pair of resistors 2| and 22 which aresubstantially identical, electrically; and one end of resistor 2| isconnected to one end of resistor 22v so that in eiect a center-tappedresistor is obtained. The other end of resistor 2| is connected byconductor 23 to one end of resistor I6, and the corresponding end ofresistor 22 is connected by conductor 24 to the opposite end of resistorI6. The common point of connection of resistors 2| and 22, which may bethe housing of flowmeter I3, is connected by conductor 25 to one of theterminals of a phase-responsive meter 28, another terminal of which isconnected by a conductor 21 to center tap l1 of resistor I6. 'Ihiscircuit will be recognized as the familiar Wheatstone bridge; and itwill thus be apparent that after a condition of balance is obtained, any

. change in: 'resistance 'of' resistors@ ".=and`'-22`=it=ii1 cause adeflection of meter 26. The remaining" terminals of meter 26 areconnected by conductors 28 and 23 tc the transformer I8 so that a.

reference voltage will be provided for phase response, necessary becauseof the use of A. C.

voltage. vThe meter 23 is designed so that it may.,V

be deflected in either direction from a central .balanced position; andif it 'is ,-properly,:cali=1- bra unbalancebetween resistors 2| and 22in 'terms of velocity of ascentor descent in feet per minute.

To change the relative resistance of resistors 2| and 22, flowmeter I3is designed so that a jet of air will impinge upon resistor 22 when theaircraft is ascending, and a jet of air will impinge upon resistor 2|when the aircraft is descending. Since resistors 2| and 22 are connectedtothe source of power I8, a current will flow through these resistorsand they will be heated thereby. The jet of air which impinges upon thecorresponding resistor will cool the latter and this change intemperature will be accompanied by a change in resistance. If thecircuit I is balanced after the resistors have reached a constantoperating temperature, a cooling of either resistor will change itsresistance relative to the other resistor and thus destroy the balanceof the circuit.

It will thus be seen that any air which ows through the owmeter I3 willcool either resistor 2| or resistor 22 and hence produce a deflection ofthe meter 26. The means whereby a flow of air in one direction coolsonly one of the two resistors 2| and 22 is described hereinafter, but itwill be seen that the direction of the flow of ar will cause the meter26 to be deflected in a direction corresponding to which resistor iscooled.

To produce the ilow of air through the flowmeter I3, the reservoir I0 isconnected through iiowmeter I3 and by-pass Il to the source of staticpressure as previously described and as the plane ascends, the staticpressure decreases and the air within the reservoir, which is at ahigher pressure, ows through flowmeter I3 to the Pitotstatic tube whereit is discharged. Since, as is well known, the pressure of the airdecreases as the altitude is increased, and since any decrease in thestatic pressure will cause a ow of air from the reservoir I0, it will beseen that as the plane ascends, the air within the reservoir will owthrough the flowmeter at a rate determined by the difference in pressurebetween the static pressure and the interior of the reservoir l0.

However, because of the change in density oi. the air as the altitude ischanged, a unit change of pressure will correspond to a smaller changein altitude at sea level than at a higher altitude.

For example, a change of pressure 0l l millimeter of mercury correspondsto a change of 36 feet at sea level; the same pressure changecorresponds to a change in altitude of 49 feet Aat an altitude of 10,000feet; at 20,000 kfeet altitude, the change corresponds to a 68 footchange in altitude; at 30,000 feet, a change in pressure of 1 millimetercorresponds to a change in altitude of 97 feet, while at 40,000 feet,this same change corresponds to 155 feet. This is illustratedgraphically in Figure 3. Assuming that the rate of iiow of air remains aconstant fora given pressure differential, if an instrument werecorrectly calibrated so as to indicate a rate of change of 100 feet perminute at sea level, the same meter with the same rate of change wouldindicate only approximately 50 feet per minute change of altif/tude at20,000 feet. This condition is obviously fi'fgtlginsatisfactory:andhence compensating means m'ust -be provided to take care of thisvariation of response with changing altitude.

Inaddition to the change of response caused by the variation of pressuredifferential at different altitudes mentioned lust above, another factorenters which is characteristic of hot wire iiowmeters. Since suchowmeters measure the 1coolil'ig'eiect of the air blowing across aresistor, Eii'."tha-air is maintained at aconstant temperature, it willbe found that the cooling effect 'is proportional to the product of thevelocity and the density of the air. Hence, since the density of the airdecreases as the altitude is increased, further correction must be made.To provide these necessary corrections, I prefer to use the meansillustrated in Figure 1.

As shown in Figure l, my preferred compensating means includes theby-pass I4 around the flowmeter I3, so that air iiowing from thereservoir I0 to the source of static pressure will generally divide intotwo streams. The first stream goes through the owmeter |3, and thesecond stream passes through the by-pass I4, later re-- joining theilrst stream intubing 34, after the first stream has completed itspassage through the flowmeter. To control the amount of air ilowingthrough the by-pass I4, I provide a valve 30 which may conveniently beoperated directly by an aneroid 3|. The valve used may be -oi.' anysuitable type," but preferably is one which may be given anydesireddegree of opening for a given displacement. If the aneroid 3| ismade of a metallic bellows which has been evacuated and Winch has aspring in it, the bellows will expand as the pressure surrounding itdecreases. It is to be understood that the outside of the bellows issubjected to the outside air pressure as measured by the Pitot-statictube as, for example, by enclosing the aneroid in a housing 32, theinterior of which is connected by tubing 33 to the source of staticpressure through tubing 34. The 60 valve 30 may be a calibrated needlevalve arranged so that when the bellows 3| is in its most expandedposition corresponding to the highest altitude for which the ilowmeteris compensated, the valve 30 will be completely closed and all airentering or leaving the reservoir I0 must pass through the flowmeter I3.Obviously, any desired linear or non-linear characteristics can be givensuch a valve by modifying the needle, the seat, or both. As the altitudeis decreased, the pressure within the chamber 32 acting upon theexterior of the aneroid 3| will be increased and valve 30 will beprogressively opened so that an increasing percentage of the air flowingthrough tube I2 may by-pass the owmeter I3 and go through valve 30. Thevalve 30 is calibrated so that the effects of changing density andchanging pressure differential will be compensated for, and the meter 26will thus give a true indication of the rate of ascent or descent.

It will now be apparent that one of the factors determining thesensitivity of the owmeter I3 will be the size of the reservoir I0, fora reservoir with a small capacity will produce a limited fiow of airthrough the owmeter I3 for a given pressure differential. Conversely, ifthe size of the reservoir is quite large, there will be considerableiiow through the fiowmeter I3 for a very small pressure differential,and it is possible to use a reservoir of such size that the resultantsensitivity is too large and the meter 26 will fluctuate with theslightest change in pressure. Such sensitivity is neither needed nordesired in aircraft installation and hence I prefer to make my reservoirof a more moderate size. A further disadvantage of using too large areservoir is found in that with a large reservoir, an appreciable timemust elapse for the pressure within the reservoir to be equaiized withthe static pressure, and this causes a lag in the indication of theinstrument. Any'delayed response or lag of ind*- cation in an aircraftinstrument is undesirable, and this is further reason for making thesize of the reservoir I as small as practicable. Other factors whichinfluence the sensitivity of the system are the voltage applied to thebridge network I5 and the sensitivity of the meter 26. Even though theinstrument panels of the modern larger aircraft are insulated againstvibration and shock, there is always a certain amount of vibrationpresent and it is thus desirable to use a ruggedly constructedinstrument. In general,

the more ruggedly constructed instruments are less sensitive. and forthis reason, the'meter 26 A will usually not be extremely sensitive. Itis thus desirable to proportion the response of the various elements ofthe system so that the reservoir I0 may be made conveniently small, themeter 26 comparatively rugged, and the voltage applied to the bridgesystem adjusted so that the desired sensitivity will result.

For the same reasons that it is desirable to maintain the reservoir I0as small as practical, it is also desirable to maintain the volume ofthe container 32 and of the tubing I2 and 33 as small as possible. Inaddition, the space on the rear side of the instrument panel of a modernairplane is very crowded and it is also quite difficult to work on theinstruments mounted thereon. Consequently, one of the advantages of mydevice is that it may be mounted Very near the Pitotstatic head wherethe length of tubings I2 and 34 will be reduced to a minimum. Conductors23, 24, and 25 may then be routed as needed, with conductors 25 and 21going to the meter 26 on the instrument panel. In this Way lengthy runsof small diameter tubing will be eliminated, and servicing andmaintenance problems will be greatly simplifled.

To secure the greatest accuracy from my improved flowmeter, it isdesirable that any change in pressure be transmitted as quickly aspossible to the reservoir I0 so that the lag of the system will bereduced to a minimumf In order to do this, it is necessary thatfrictional and turbulent losses in the passage of the air through theflowmeter I3 be reduced to a minimum. While iiowmeters have previouslybeen designed which make use of this hot wire resistance principle andprovide the desired differential effect, they have generally had arather elaborate and labyrinthian passage for the air which causedexcessively large losses due to friction and turbulence of the air. Inorder to overcome these difficulties I have provided the flowmeter I3shown in detail in Figure 2. As illustrated there, I provide a housinghaving a partition 4I therein which divides the housing into twocompartments 42 and 43. Suitable connecting means 44 and 45 permit thetubings I2 and 34 to be connected to the compartments 42 and 43 so thatcompartment 42 is fluid connected to reservoir I 0 and compartment 43 isfluid connected to the source of static pressure. An orifice 46 in thepartition 4I provides a passageway for air between compartments 42 and43, and as is Well known, such an orifice produces a jet extendingdownstream from the orifice when fluid is passed therethrough. Upstreamfrom the orifice 46 there is no region of increased rate of ow exceptimmediately adjacent the orifice, and this latter zone is guite small incomparison to the size of the jet on the downstream side of the orice.

Immediately adjacent the orifice 46 and located so as to be within thejet therefrom, I mount a resistor 50 which may be of any suitable formand material, but which from the nature of the problem, must berelatively small. By way of example only, I have found that thefilaments of small incandescent light bulbs, such as those having aminiature candelabra base and a glass envelope of approximately the samesize, make excellent resistors when the glass envelope has been removed.Such resistors are relatively sturdy and are manufactured to relativelyclose tolerances both electrically and mechanically. In addition, thebase of the bulb makes a very convenient mounting arrangement for theresistor since it provides the necessary mechanical rigidity andstrength while permitting easy interchangeability should a resistor bebroken for any reason whatsover. Consequently, in Figure 2 I have showna socket 5I, adapted to receive the base of such a light bulb, andmounted on a spider 52 so that the resistor 56 is rigidly held andproperly supported with respect to the orifice 46. Since the outer shellof such sockets usually forms one connection to such light bulbs, I havefound it convenient to make the spider 52 of metal so that the housing40 may become one of the connections to the resistor 50. It Will beapparent, though, that if for any reason it is desired that the housing40 be completely insulated from the resistors, thelspider 52 may be madeof insulating material and separate connections run from the socket 5I.The other connection from the socket 5I may be carried to any suitableconnector 53 properly insulated from the housing 40.

Similarly, in compartment 42 a resistor 60 is mounted in a socket 6Iwhich is supported by a spider 62, while one end of the resistor may beconnected to connector 63 which is likewise suitably insulated fromhousing 40. seen that one connection from each of resistors 50 and 60 isgrounded to the housing 40 and a suitable connection 64 may be providedto make connection thereto. If, of course, it is desirable to have thehousing 40 electrically isolated from the system, the connector 64 maybe insulated from the housing and separate leads from each of th-eresistors 50 and 60 run thereto.

It has been found that the size of the orice 46 and the distance whichthe resistors 50 and 60 are mounted therefrom is not particularly Itwill thus be Y critical so long as the resistors are mountedsubstantially equidistant from the corresponding faces of the partition4|. The bulbs from which the resistors 50 and 60 are secured are held tovery close tolerances as to the distance of the resistor from the baseof the bulb, and if the spiders 52 and 62 are mounted so that thesockets 5i and 6|,.respectively, are equidistant from the partition 4l,the amount of unbalance in the bridge circuit I5, caused by replacingresistors, will be practically negligible for all normal measurements.It will be apparent, of course, that the resistors 50 and 60 should notbe located so close to the partition 4I that there is any danger oftheir touching it, or of the insulation of the air breaking down at highaltitude so that an arc is established betweeny the resistor and thepartition. Similarly, the resistors should not be mounted so that theyextend into the zone of relatively high velocity air on the upstreamside of the oriiice 46, but this last requirement Will generally betaken care of when the previously mentioned requirements are satisfied.

It will thus be seen that I have provided a fiowmeter having directionalcharacteristics which has a straight line flow therethrough, and

only a single orifice, thereby creating a minimum of turbulence andreducing frictional losses by an appreciable degree. In addition,because of the simplicity oi design, this flowmeter may be manufacturedat a much lower cost than those previously proposed which required aconsiderable amount of special and delicate machine and tool work.

, It will be seen that'by using a owmeter which measures the flow of airfrom a reservoir, increased sensitivity with greater mechanicalruggedness may be provided than if a diaphragm type of meter were used.With my device, by increasing the size of the reservoir, the sensitivitymay be greatly increased without impairing' the ruggedness ofconstruction; while with the diaphragm type of instrument, wherein apressure differential causes a diaphragm to move and this movement isindicated by a needle mechanically connected thereto, an increase insensitivity requires either a change in the'mechanical advantage of theconnecting mechanism or requires a change in the size of the diaphragmwhich likewise leads to many complications from the construction andmaintenance standpoint.

It will thus be seen that I have provided an improved fiowmeter which iscompensated for the condition of the iluid which it is measuring, andWhile I have shown and described a preferred form of my invention, I donot Wish to be limited to the particular form or arrangement of' partsherein described and shown except as specifically indicated by myclaims.

I claim as my invention:

l. In a device of the class described, in combination, hot wire owresponsive means, said means comprising an enclosure divided into twocompartments by an apertured partition and including a resistor in eachcompartment aligned with said aperture and spaced therefrom, firstconduit means for connecting one of said compartments with a source ofstatic pressure, an enclosed reservoir, additional conduit meansconnectingr said reservoir to the other of said compartments, by-passmeans connecting said first conduit means and said additional conduitmeans. said by-pass means including valve means for controlling flowthrough said by-pass means, pressure responsive means for operating saidvalve means, means enclosing said pressure responsive means, and meansfor placing said enclosing means in fluid flow relation to the firstconduit means.

2. In a rate-of-climb indicating device for an aircraft, in combination,an enclosed reservoir, a hot wire flow sensing means, conduit meansconnecting said reservior and sensing means in series with a source ofstatic pressure, additional conduit means connected to said first namedconduit means in by-pass relation with said flow sensing means, valvemeans for controlling ow through said additional conduit means, andmeans responsive to thepressure of said static pressure source for`operating said valve means in a manner to compensate said flow sensingmeans in accordance with variations in said pressure.

3. An instrument of the class described which includes: a chamberthrough which iiuid may ow; a first resistor mounted within saidchamber; a second resistor mounted within said charnber and close to butnot touching said lirst re-` sistor; a partition member having anorifice therein interposed between said first and said second resistorswith said orifice in alignment with said resistors whereby when iiuidflows through said chamber, said orifice concentrates said iiow of uidon the resistor which is downstream of said oriiice and providessubstantially no concentration of said flow of fluid on the resistorwhich is upstream of said orifice; conduit means connected tosaid-chamber in by-pass relation to said orifice; valve means forcontrolling flow through said conduit means; and pressure responsivemeans connected to said conduit means for actuating said valve means.

4. In a device of the class described, an enclosed fluid reservoir,conduit means for connecting said reservoir to a source of staticpressure, said-conduit means including a hot wire iiow responsive means,additional conduit means connected to said first named conduit means andby-passing said flow responsive means, valve means for controlling iiowthrough said additional conduit means, and pressure responsive means foractuating said valve means, said pressure responsive means beingconnected and arranged for responding to said static pressure.

- GEQRGE L. BORELL.

REFERENCES CITED The following references are of record in the file ofthis patent:

